Electric wave amplifying system



Patented Sept. 27, 1938 UNITED STATES PATENT OFFICE ELECTRIC WAVE `AMPLIFYING" SYSTEM Albert L. Stillwell, Westfield, N. J., assgnor to Bell Telephone Laboratories,

Incorporated,

This invention'relates to wave amplifying'systems.

An object of the invention is to adjust feedback in an amplifier, for example for gain adjust- ;ment o-r transmission equalization, without-producing undesired variationof the amplifier input or output imped-ance.

One specific aspect of the invention is an amplifier having two impedances connected alter- Y .nately between its grid and load circuits andv also alternately between its incoming and plate circuits. (Connecting'two impedances alternately between two circuits signifies connecting one of the impedances, one of the circuits, the other Vimpedance and the other circuit successively in a closed loop, the impedances thus alternating as to their connection between the circuits.)

The impedances may be caused to vary inversely, to control feedback in the amplifier, for example to vary negative feedback for adjusting gain or equalizing transmission while maintaining constancy of the amplifier impedances faced by the load and incoming circuits.

With sufficient feedback these amplifier impedances can be made to approach asymptotically a constant resist-ance (matching the resistances 0f the attached load and incoming circuits, re-

spectively) even when their valuesv without feed-'f back would vary considerably with frequency, for

example because of variation with frequency in transmission efficiency of input and output trans-f formers in the grid and plate circuits, respective- In another specific aspect `the invention is an amplifier with two feedback circuits, each symmetrical or balanced with respect to ground, and

each by its feedback, affecting the gain in the same sense as the other, but feedback through one tending to produce a given variation of the .amplifier input or output impedance .withfre-V' quency and feedback through the other tendingy to produce a complementary or neutralizing variation, so that the amplifier` input and output impedances are unchanged upon variation of.v frequency. For example, the feedback through one feedback circuit may be series-series negative feedback and the feedback through the otherl may be shunt-shunt negative feedback, and two inverse reactive networks may be connected in the two feedback circuits, respectively, for giving a desired variation of the amplifier gain'with frequency while maintaining the amplifier input and output impedances constant.

Other objects and aspects of the invention willv be apparent from the followingdescription and claims.

Fig. 1 is a schematic diagram of an amplifier circuit embodying a form of the invention;

Fig. 2 shows a modified form of amplifier circuit; and Y Figs. 3 to 5. show specific forms of networks that may be used as imped-ance elements in circuits such as those ofFigs. 1 and 2. E

. he amplifier shownV may be, for example, stabilized feedback amplifier of the general type in which a portion of the output wave is fed back in gain-reducing phase and in amount sufficient to reduce distortion below the distortion'level without feedback. Such feedback is disclosed,

for example, in the copending application of HL S. Black, 606,871, filed April 22, 1932, for Wave translation system, now Patent No. 2,102,671, Dec- 21, 1937,. and in the article by Black on Stabilized feedback amplifiers, published in Electrical Engineering, January 1934, pages 114 to 120.

The amplifier I is shown as of the Vacuum tube type and may have a single stage or any desired number of the-tandem connected stages, G and P designating the grid of the first tube and the plate of the last tube. The amplifier may have in its input circuit an input transformer 2, and may have in its output circuit an output transformer 3. Waves to be amplified by the amplifier are supplied by incoming line ork circuit L; and the amplified waves are Atransmitted to outgoing line or circuit L. The amplifier circuit has -two feedback paths Zn and Z21,'shown as two-ter.-

minal transmission control networks of generalized impedances. The amplifying path of the amplifier may be referred to as the ,ii-circuit, and the feedback circuits may be referred to as the -circuits, or the ,B1-circuit and the pf2-circuit,

respectively, the significance of ,ul and being as indicated in the Black application and article mentioned above.

The feedback through path Zn may be, forV and ingeneral they will be cheaper to build and will be able to provide much higher gains when desired. v

It is seen that the path Zai is in serial relation to the line L' with respect to the amplifier input and is in serial relation to the line L'with respect to the amplifier output. Thus, the feedback through path Zzl is a series feedback at the input side of the amplifier and aseries feedback` Therefore,

at the output side of the amplifier. the feedback through path Z21 will be -referred to as av series-series feedback, and p-ath Zzi will be referred to as a series-series feedback patho circuit.

It is seen that the path Z11 is in shunt relation to the line L' with respect to the amplifier input and is in shunt relation to the line L with respect to the amplifier output. Thus, the feedback through path- Z1: is a shunt feedback at the input side of the amplifier and a shunt feedback at the output side of the amplifier. Therefore,

the feedback through path Z11 will be referred to as a shunt-shunt feedback, and path Z11 will be referred to as a shunt-shunt feedback path or circuit.

The amplifier circuit forms a bridged Tnetwork connecting lines L and L. The transformers 2 and 3 are two of the arms ofthe Tyand they are bridged by an arrn Z11 of the bridged T network, Thethird arm, of: the Tis the arm; Z21. Whenever, in, theA passive condition of: the.. amplifier circuit, each, 0f the four arms of the bridged T network hasitsimpedance equal to that of each of the circuitsL and LI, (or in otherwords, whenever. the primary-to-secondary. impedance of transformer 2., the secondary-to-primaryimpedance of. transformer 3, the impedances ZL and ZL of the lines L'; and: L, and the im.- pedances Z11 and Zv21 areV all equal: and, indeed, whenever Z11.Z21 isrequal. tothe product of. the

impedance of L andthe impedance of the am-.. plifier output circuit and also equal. tothe prod. uct of the impedance of the load circuit L and'.

the impedance of. the amplifier input; circuit, then the system is in a condition which may be called the condition vof passive ba1ance. In. this condition, conjugaciesexist between. the incom.- ing circuit L' or source of waves to be amplified and the output transformer 31er amplifier outputl circuit, and between the loadycircuit or receiv... ing circuit L and theamplifier input circuit orinput transformer 2, and.y between impedance Z21 and impedancev Z11. Thus, these conjugacies can be maintained even ifv Z21v and Z11` are varied, provided` they are varied inversely (i. e., so that their product remainsy constant). Further, with considerable" amounts of. negative feed-back inthe amplifier and relatively largeV value` for Z11, theseconjugacies` practically are n maintained even when the values of theamplifier inputandoutput impedances without feedback vary widely from their values for the-passive conditionof passiveA balancefthe feedback 50.L tending to-restore the. conjugacies andmaintain the amplifier input and output impedances ZFA respectively; and then ifi Z'L and Z1. are equaly resistances Re, Z'F and. ZF. each. substantially equals R0.

Thus, the impedances Z11 and, Z21Y may be Caused to vary inversely, tocontrolfeedback iny the amplifier, for example, for adjusting gain .or equalizing transmission4 while` maintaining constancy ofA the amplifier input and output imV pedances facedby the incoming circuit andtheload.

negative feed-A With considerable negative feedback the amplifier input and output impedances can be made to remain respectively substantially equal to the impedances of the incoming and outgoing lines, notwithstanding inverse variation of Z11 and Zai andl notwithstanding variation of the amplification from the amplifier input circuit to the amplifier output circuit (due, for example, to variation of the impedances or amplification constantsof the amplifier tubes as a result of changing tubes or types of tubes, or to variation of the transmission eiiiciency of transformers 2 and 3 withA frequency).

Z11 and Zn may be inversely variable resistances when they are to be used for gain adjustments in; which the gain changes are the same for all frequencies in the frequency range of interest and ZL and Z1. are resistances.

When'Zn and Z21 aretoserve for varying amplifier gain. with frequency, for. example, for

transmissionY equalization, they may be suitableA reactive inverse impedances, for instance, as

shown-in Figs. 3to 5 described. hereinafter. With considerable negative feedback in the amplifier, theV amplifier gain indecibels will be and output bridges, these must be designed to.

minimum feedback loss to allow maximum feed- .back This corresponds tomaximum line loss which ispresent alsofor maximum gain.4 However, with. the equalizerin accordance with this invention the maximum gain corresponds to maximum feedback. loss and hence minimum line loss. For a 30 decibel maximum` gain requirement` the corresponding. line loss would be of the order of; 0.3.decibel; for example.

The amplifier. circuit ofFig. 1 isv unbalanced with respect to ground. That is, one side can be groundedso that there are no longitudinal transmission, problems.

Fig. 21 shows a. modified form ofl the circuit which is balanced with respect to ground. That. is,eachf side oft the line isabove groundby the same impedance. is showndivided. into two equal parts, and the impedance Z21 isshown connected between the twohalves of. the'primary winding. of the input.

transformer 2 and. also; connected between` theA two halvesofthe secondary winding ofthe out.- put transformer 3. As inthe case of. Fig. 1, with Z11-Zz1 equalV to ZL-ZL, the amplifier input impedance is.

Z11Z21 V Z F. ZL

andthe amplifier outputv impedance is Z -Z t/L2! Where the gain of the feedback amplifierf is to-be varied by different amounts at different frequencies, for example, to compensate for` changes; of; line attenuationzwhich. varyy with` frequency, this may be accomplished, for example,

In,Fig.;2.the impedance Z11 by adjusting continuously variable reactances,

such as air condensers or variometers or both, v

in inverse networks Zn and Zn, while maintainingtherelation Z11Z21= (R102 0r Z11Z21=Z'LZL. For instance, the networks Zu and Z21 of Fig. l or Fig. 2 may be composed of resistances, inductances and capacities, with the capacities of network Zn adjustable and the inductances of network Z21 adjustable, as indicated in Fig. 3. adjustments may be made manually; or they may be made automatically, for example, by pilot wire or pilot channel controlled operating means in response to effects produced by temperature or weather changes that produce the line attenuation changes.

Figs. 4 and 5 show examples of another type of variable network that can be used for the networks Zn and Z21, for instance, in the manner just described for the networks of Fig. 3. This typeisa terminated bridged T constant impedance structure, which can be used as a two terminal network, for example, in the bridged T network of Fig. l or Fig. 2. In the networks Zn and Z21 of Figs. 4 and 5 the inverse impedances are designated Zn and Zzl. rI-he impedances designated K in these networks have the value K, K being a complex quantity, and Z11-Z21'=K2. The amplier gain adjustments, for example, for compensating for line attenuation changes that vary with frequency, may be made for instance, as indicated by the arrows in Figs. 4 and 5. These figures show the networks Zn and Zar composed of resistances, inductances and capacities; and in Fig. 4 the arrows indicate a capacity in network Zn and an inductance in network Z21 as adjustable; and in Fig. 5 the arrows indicate two of the capacities of network Zn and two of the inductances of network Z21 as adjustable. chosen to give a prescribed or desired average slope of the amplifier gain-frequency characteristic for the normal setting of these adjustable elements, and then the characteristic can be raised and lowered by adjustment of these elements. If necessary for obtaining the desired characteristics, resistance as well as reactance elements of the networks Zn and Zzi can be made adjustable. f

What is claimed is:

l. Wave amplifying means having input and output circuits, a wave source for association with said input circuit, a load circuit for association with said output circuit, and two irnpedances connected alternately between said input and load circuits and also alternately between said source and said output circuit, said load circuit being in shunt relation to one of said impedances and in serial relation to the other impedance, with respect to said output circuit, and said source being in shunt relation to said one impedance and in serial relation to the other, with respect to said input circuit.

2. The method of operating a transmission controlling system including an ampliiier and two impedance networks connected alternately between its input and receiving circuits and also alternately between its output and sending circuits, which comprises causing the impedances of said networks to vary inversely.

3. A system comprising wave amplifying means having an input circuit and an output circuit, a wave source and a load circuit, and means connecting said source and said load circuit in bilaterally transmitting relation to each other with said source in conjugate relation to said The The impedance product K can be,

two transmission circuits, and means connecting one in conjugate relation to said output circuit and connecting said input circuit in conjugate relation to the other, said means comprising passive means connecting said two transmission circuits in transmitting relation.

6. A system comprising wave amplifying means having an input circuit and an output circuit, a wave source and a load circuit, and a single bridge circuit including said source and said circuits and connecting said output circuit in conjugate relation to said source and said input circuit in conjugate relation to said load circuit.

7. A wave amplifying system comprising a bridge circuit composed of six branches, an amplier having its input and output circuits respectively connected in two o-f said branches having a common junction point, and a source o f waves to be amplied by said amplier and a load circuit for said amplier respectively connected in two others 0f said branches having a common junction point', the branches including said input circuit and said source having a 'common junction point, and the branches including said output circuit and said load circuit having a common junction point.

8. A bridge circuit composed of six branches, an ampliner having its input and output circuits respectively connected in two of said .branches having a common junction point, and

respectively connected in two of said branches having a common junctionk point, a source of` Waves to be amplified by said amplifier and a load circuit for said amplifier respectively connected in two others of said branches having a common junction. point, the remaining two branches respectively consisting of two inversely adjustable impedances, one of said remaining two branches connecting said input and output circuits and the other connecting said two junction points.

10. A bridge circuit composed of six branches, an amplifier having its input and output circuits respectively connected in two of said branches having a common junction point, and a source of waves to be amplified by said amplier and a load circuit for said amplier respectively connected in two others of said branches'having a common junction point, the product of the iming two of said-six branche's,one .of ls'aidre'- maining two branches connecting said `inputzand output circuits and the otherA connectingsaid two junction points` 11. A bridge circuit composed of six branches, an amplifier having itsinput and output circuits respectively connected'in two of saidbranchesy having a common junction point, and a sourcev of waves to be amplified by said'amplier-anda -load circuit for said amplifierrespectively con nected in two others of said branches having a common junction point; one-of 4the 4latter two branches having its impedance equal tothe square root of theY product of the impedances of 15 ithe two remaining branches, one'. of said-:two

of waves to be amplifiedV by-said amplifier and# a load circuitfor said kamplifier respectively con-1I 'nected in two others of saidbranches having a common junction point, Veach of the latter tw-o'I branches having its Yimpedance-equal to a given resistance, and the remaining tw-o branches having the product of their impedances equalto the v square of said resistance, one -of Vsaid-remain-l.

ing two branches connecting said inputand out- Y put circuits andthe other connecting said .two l,

junction points.

13. A wave amplifying system comprising a bridge circuit composed 'of six` branches, an am-` plifier having its input and output circuits respectively c-onnected in two of said branches Vhaving a common junctionv point,` and a source of waves to be amplied by saidamplier anda cload circuit for said amplifier respectivelyA connected in two others'of said branches-having. a common junction point the remaining two branches providing feedback that changes 4the gain of the system for transmissionfrom. said w and output circuits and having input and outsource to said load circuit without changing the impedances presented to said source-:and said load circuit,` one of ysaid remaining two branches connecting saidinput tandoutput circuits-and the other connecting said two junction pointsV m; 14. A bridge network comprisinga line having a frequency selective transmission 4characteristic 1 and having sending and receiving sectionsand an amplifier circuit. connecting said sections, said amplifier circuit comprising an amplifier. with" fi'nput and output circuits connected for transmitting waves from said output circuit to said input circuit that produce variation of gain of said am plier circuit compensatingfor variation of theline attenuation with frequency, and said kbridge *network having said output-section in one` of its armsfandhaving 'said input section in fa branch` connected across that arm and an adjacentarm.

15.'y The combination of a wave source, a load circuit, and-a bridged T network for connecting saidsource lfor transmission through said network l'to said load circuit, said network comprising ,an amplifier and having the amplifier input circuit as one arm, the amplifier output circuit as an adjacent arm, and two inversely variable impedances asy the remaining arms, one connected across the amplifier input and output circuits, and one connected from the junction of the amplifier input and output circuits to the junction of the source and the load circuit.

16. In a transmission system, a transmitting medium having a varying efficiency dependent uponv the frequency of the wave transmitted therethrough, an amplifier for off-setting at least a portion of the transmission loss of said medium, said-amplifier having input and output circuits, incoming. and outgoing lines for connecting said amplifier-into said medium, and a network comprising said .circuits and two general impedances, one ofzsaid circuits and one of said impedances being vvserially connected across one of said lines, the other circuit being bridged across said one circuit and the other of said general impedances in serial relation', the other of said lines being connected across the whole of said network, the network being designed to simulate the Varying efficiency of at least a portion of said medium and match the attached impedances of said lines.

17. A system comprising wave amplifying means having an input transformer and an output transformer, a wave source and a load circuit,.and means connecting said source and said load circuit. in bilaterally transmitting relation toeach other with said source in conjugate relation to said output transformer and in transmitting relation to said input transformer, and with said output transformer in transmitting relation to said input transformer and said load Y circuit.

18. A negative feedback amplifier having input put impedances, a wave source attached to said input impedance, a load circuit attached to said output impedance, and two inversely variable impedances with their product approaching that of said input and output impedances, one of said inverse'impedances and the load circuit being in serial relation with respect to the amplifier output circuit, and the amplier input circuit and said one inverse impedance being in serial relation with respect to the source, and the source, in parallel with the amplifier input impedance and said one inverse impedance in series, being connected across the other inverse impedance and the load in series.

ALBERT L. STILLWEIL. 

